In cellular networks for radio communication with user controlled terminals, commonly referred to as User Equipments, UEs, interference may occur between different transmissions made at the same time and on the same frequency band. For example, in order to increase capacity in networks employing Long Term Evolution, LTE, so-called co-scheduling is enabled for uplink transmissions of signals from multiple UEs in a cell where the UEs in the cell can be scheduled to transmit basically at the same time and on the same bandwidth, by using more or less orthogonal signals. The term “orthogonal” implies that the signals are basically non-interfering with each other. Still, interference typically occurs for data signals to some extent, both between co-scheduled UEs in the same cell and between UEs in different cells, since it is not always possible to make simultaneous transmissions from different UEs completely orthogonal, either within a cell or between neighbouring cells. Data signals are typically transmitted on a Physical Uplink Shared Channel, PUSCH.
Some examples of network scenarios where interference can potentially occur include when UEs are located close to the cell border or “cell-edge”, when large cells are divided into multiple adjacent sectors, when pico-cells are deployed within the coverage of a macro-cell, and when a hotspot access point serves a small area with high data throughput. FIG. 1 illustrates an example with two neighbouring cells, a first cell A and a second cell B, with radio coverage by a first base station 100A and a second base station 100B, respectively. In the first cell A, a first UE 102 and a second UE 104 transmit respective uplink data signals x and y simultaneously on a shared bandwidth, which may thus interfere with each other when received by the first base station 100A. The figure also illustrates that a third UE 106 in the second cell B transmits an uplink data signal z on the same bandwidth, which may be interfered by the transmission from the second UE 104 when received by the second base station 100B, as indicated by a dashed arrow y′.
Typically, an interfering data signal such as y′ is a disturbance that makes it difficult to detect the interfered data signal z properly at the second base station 100B, although solutions have been developed for data signals where the interfering signal y′ is transformed into a useful signal by base station 100B for decoding the data signal y coming from UE 102 in base station 100A. In general, LTE networks can be designed to use Coordinated Multipoint Processing, CoMP, where base stations of different cells and/or sectors operate in a coordinated way for detection of data signals and scheduling. An example of uplink CoMP is when a data signal transmitted from a single UE is received and jointly processed at multiple reception points, e.g. base stations, in order to improve the link quality.
In this context, the receiving base station may have functionality to estimate the uplink radio channel used by a transmitting UE, to support and facilitate signal demodulation and detection on that channel, e.g. PUSCH. To this end, the UE sends a reference signal known as the “Demodulation Reference Signal”, DMRS, that the base station can use for performing channel estimation. The channel estimation is then employed by an equalizer in the base station for demodulation of received uplink data transmissions, e.g. on the PUSCH. The DMRS is thus typically associated to the PUSCH used. In LTE, a radio frame scheme with 10 subframes of two slots each is used, and two DMRSs are typically transmitted in a subframe, with one DMRS in each slot. The DMRS has the same bandwidth as PUSCH and may be precoded in the spatial domain in a similar way as data transmitted on the PUSCH. Achieving orthogonality of DMRS transmissions from co-scheduled UEs will allow for improved accuracy of the channel estimation. The equalizer in the receiving base station is then able to separate, e.g. using multi-antenna techniques, the co-scheduled DMRS transmissions, and even to suppress interference at the base station's receiver.
However, DMRSs transmitted at the same time from different UEs may potentially interfere with each other, either within a cell or between neighbouring cells, e.g. as explained above. Different techniques have been introduced in different releases of LTE to achieve orthogonal or “semi-orthogonal” DMRSs, thus limiting the level of interference between them to allow accurate channel estimation. It is typically assumed in LTE that DMRS transmissions from different UEs should be orthogonal within each cell and semi-orthogonal between neighbouring cells. As a result, a DMRS transmitted in one cell may be interfered by a semi-orthogonal DMRS transmitted at the same time in a neighbouring cell, thus disturbing the channel estimation in the former cell.
A DMRS can be defined by a base sequence and a cyclic time shift of the base sequence such that the DMRS to be transmitted is generated as a function of said base sequence and based on the applied cyclic time shift in a manner well-known in this field. According to releases 8, 9 and 10 of LTE, the base sequence of a DMRS is cell-specific by being a function of the cell identity, as well as other cell-specific parameters. Further, some DMRSs generated from different base sequences of different cells can be considered semi-orthogonal when transmitted simultaneously. The base sequences employed in LTE can be chosen based on various properties, e.g. the so-called low cross-correlation absolute value between different base sequences. Because of this property, using different base sequences for DMRSs can cause relatively low mutual interference from the DMRSs, even without being perfectly orthogonal, hence the term “semi-orthogonal” which corresponds to the sometimes used term “pseudo-orthogonal”.
DMRSs generated from the same cell-specific base sequence can be made orthogonal by applying different cyclic time shifts on that base sequence to provide circular rotation in the time domain, which method is often referred to simply as “CS”, and is used in LTE where there are currently 12 different CS values available. Even though CS with cyclic time shift is effective for limiting interference between simultaneously transmitted DMRSs used for channels with completely overlapping bandwidths, full orthogonality can be lost when the channel bandwidths differ and/or when UEs employ different base sequences. So-called “CS hopping” is another method that can be used for reducing the impact of interference between simultaneous DMRS transmissions, where the CS value is changed over time according to a hopping pattern which is configured per cell.
It is generally useful to spread out and “randomize” the interference to limit its impact on link quality. In order to increase interference randomization, a pseudo-random offset is applied to the CS values when using the CS hopping method. A different CS offset is usually applied in each slot and this CS offset is known at both the UE and the base station, so that the CS offset can be compensated at the receiving side during channel estimation. The pseudo-random CS offset is combined with a signaled CS offset for each slot, and a “modulo 12” operation is performed in order to avoid exceeding the maximum CS value of 12. Typically, CS randomization is always employed and generates random cell-specific CS offsets per slot. The pseudo-random CS pattern to use is determined by a function of the cell-ID and other cell-specific parameters.
In LTE release 10, cyclic time shift is used in conjunction with a method known in the art called Orthogonal Cover Codes, OCC, which is a multiplexing technique where different orthogonal time domain codes are applied on the two DMRSs transmitted in an uplink subframe. For example, a first OCC code denoted [1-1] can be applied on one DMRS transmission to suppress another interfering DMRS transmission as long as its contribution after passing through a matched filter in the base station is identical on both DMRSs of the same subframe. Similarly, a second OCC code denoted [1 1] is able to suppress an interfering DMRS provided that its contribution after the matched filter has an opposite sign respectively on the two DMRSs of the same subframe. Virtually full orthogonality between two UEs can thus be achieved by applying different OCC codes on their DMRS transmissions only if the same base sequence is used on the DMRS in both slots by each UE.
However, as the above-mentioned network scenarios are sensitive to interference and will be more commonly deployed, and as CoMP will be extensively used for uplink transmissions, the requirements for effective channel estimation will become even greater to achieve acceptable link quality. It is thus a problem that the interference between DMRSs cannot be limited sufficiently in situations of dense traffic and/or closely located UEs.